Previous studies show the classical approach of using only negative feedback in controlling dynamic and steady-state performance of control systems. See, for example, N. K. Sinha, "Control Systems", Holt, Rinehart and Winston, 1986, pp. 59-70, G. H. Hostetter et al, "Design of Feedback Control Systems", Holt, Rinehart and Winston, 1982, pp. 3-5, B. C. Kuo, "Automatic Control Systems", Prentice-Hall, INc., 4th Ed., 1982, pp. 3-16.
In the field of drive systems including ac drives this classical approach is maintained as seen from G. P. Dubey, "Power Semiconductor Controlled Drives", Prentice Hall, INc., 1989, pp. 275-278, 313-319, 335-338, 342-345, and 379-381, W. Shepherd & L. N. Hulley, "Power Electronics and MOtor Control", Cambridge University Press, 1987, pp. 261-264, and M. H. Rashid, "Power Electronics: Circuits, Devices, Applications", Prentice-Hall, Inc., 1988, pp. 364-369, and 376-378.
In the particular area of high-performance ac synchronous and induction motor drives a complex algorithm of field-oriented (vector-controlled) motor control had been used to control the ac motor developed torque and reduce the effects of load torque disturbances. In addition to being complex, which directly contributes to the complexity of an ac drive system of high performance and thus to the system costs, the algorithm does not provide load independence.
The ultimate load regulation performance in drive systems should be invariant to the change of load and limited only by the physical properties of the system, such as the finite energy level of available sources, the finite power dissipation capability of available components and the finite speed of transition of control signals. Such an ultimate performance has not been achieved using the classical approach of negative feedback and augmenting the control algorithm with advanced nonlinear/adaptive techniques.